Conventionally, materials are classified as metals, semiconductors, or insulators according to their ability to conduct electricity. In a material, electrons are organized in discrete energy levels or bands separated by a distinct amount of energy. According to band theory, if the highest filled band is only partly full, the empty states will assist conduction. The energy required to promote an electron from one energy band to the next higher band is called the band gap energy. Its magnitude determines whether such a material is a metal, semiconductor, or insulator. The energy level at the midpoint between the two bands is termed the Fermi level.
In metals the partially filled upper band is referred to as the conduction band. Addition of small amounts of energy excites electrons in this level quite easily. These easily excited electrons are responsible for the electrically conducting nature of metals. For a semiconductor, the valence band is completely filled, and the conduction band is completely empty. Therefore, exciting an electron requires the addition of energy equal to that of the band gap energy, approximately 1 eV at room temperature. Similarly, insulators have a completely filled valence band and a completely empty conduction band. However, the band gap energy required to move an electron into the unfilled conduction band is much greater than that of a semiconductor, on the order of 15 eV. Insulators, therefore, do not conduct electricity except under the application of rather large voltages.
Although most polymers are insulators, a class of inherently conductive polymers (ICPs) exists that cannot be classified in any of the above categories. Through oxidation and reduction reactions, ICPs are doped to electrically conductive states. The radical cations and radical anions formed in these reactions are accompanied by a distortion or relaxation of the polymer lattice which acts to minimize the local strain energy. The energy level associated with these distortions is split from the continuum of band states and symmetrically positioned about the Fermi level.
ICPs can be divided into two groups, those possessing degenerate ground states and those without degenerate ground states. ICPs with degenerate ground states, e.g. polyacetylenes, do not have a determined sense of bond alternation. In these materials, the transposition of single and double bonds yields energetically equivalent structures. Most ICPs, such as poly(p-phenylene), are non-degenerate. In these materials, the transposition of single and double bonds leads to the formation of quinoid structures of significantly higher energy than the parent aromatic forms.
The level of conductivity achieved in ICPs depends on the molecular structure of the polymer backbone, the degree of doping, and the nature of the counter ion species incorporated. Conductive polymers display an impressive range of electrical conductivity produced by controlled doping. The considerably larger conductivity range in ICPs compared to semiconductor crystals results from the intrinsic difference in their structures. Because of their rigid, three dimensional lattice structure, inorganic semiconductors can only accept dopant ions at low concentrations and therefore have a limited conductivity range. ICPs, on the other hand, consist of an assembly of pseudo-one-dimensional conjugate chains. They are able to accept far more dopant ions, thereby achieving a greater range of conductivity.
Pyrrole is polymerized by an oxidative process. Polypyrrole can be prepared either chemically through solution processing or electrochemically through polymer deposition at an electrode. Both processes involve electron transfer. The polymerization proceeds via the radical cation of the monomer which reacts with a second radical cation to give a dimer by elimination of two protons. Dimers and higher oligomers are also oxidized and react further with the radical cations to build up the polypyrrole chain. The polymer is thus formed by eliminating two hydrogens from each pyrrole unit and linking the pyrroles together via the carbons from which the hydrogens were eliminated.
Pyrrole is readily polymerized by a wide variety of oxidizing agents in aqueous solution. Polypyrrole can also be prepared electrochemically. Typically, polypyrrole films are galvanostatically deposited on a platinum electrode surface using a one-compartment cell containing an aqueous solution of pyrrole and an oxidizing agent.
Although polypyrrole is prepared in its oxidized conducting state, the resulting polymer can be subsequently reduced to give the neutral, highly insulating form. Electrochemical switching between the conducting and insulating state is accompanied by a color change from blue-black to yellow-green and a conductivity change which spans about ten orders of magnitude. As with polyaniline, switching between conducting and insulating states is a reversible process.
Conductive polymers have traditionally been plagued by problems of stability, narrowly defined here as the maintenance of conductivity. In the process of oxidative doping, ICPs are stripped of a fraction of their electrons, thereby increasing their conductivity by several orders of magnitude. While the gaps left by the lost electrons provide a pathway for charge to be conducted down the polymer chain, they also make the polymer highly reactive with oxygen and water. Stabilization, then, becomes an effort to minimize doping site loss by chemical degradation or doping site quenching by such contaminants as oxygen or water. Various methods have proven effective in stabilizing ICPs; among these are encapsulation techniques and the use of barrier resins and sacrificial layers.
Compared to other conjugated polymers, polyaniline and polypyrrole have an unusually good chemical stability and encounter only a mineral loss of conductivity upon exposure to ambient environments. For example, it has been found that the conductivity of emeraldine hydrochloride formed by the protonation of emeraldine base did not change during extended periods in laboratory air. Similarly, the electrical properties of polypyrrole are indefinitely stable in air at room temperature.
Because ICPs form rigid, tightly packed chains, they are generally resistant to processing, a problem which has limited their widespread commercial use. While tight chain packing is essential for interchain charge hopping, it also prevents the polymer from intermixing with solvent molecules. Therefore, as a whole, ICPs tend to form as intractable masses. Many approaches to synthesizing tractable ICPs have been explored including substituted derivatives, copolymers, polyblends, colloidal dispersions, coated latexes, and ICP composites. These efforts have yielded a rich variety of blends, random copolymers, and graft and block copolymers with enhanced processability.
For many years, researchers have strived to prepare smooth, coherent films of polyaniline and polypyrrole. In 1968, cohesive polypyrrole films were electrochemically prepared at an electrode surface. The electrochemical preparation of free standing polyaniline films with a fairly smooth, featureless topography was accomplished in the early eighties. Unfortunately, ICPs formed by electrochemical polymerization are generally insoluble and brittle.
In an effort to produce conductive polymer films with improved mechanical properties, researchers have attempted to synthesize ICPs on polymeric supports. Because such supports are normally electrical insulators, the standard electrochemical methods of deposition are difficult to apply. Most research, therefore, has centered on the chemical polymerization of ICPs on suitable substrates.
For example, polypyrrole films have been formed on the surface of a polyvinyl alcohol-ferric chloride (PVA-FeCl.sub.3) complex. An aqueous solution containing a mixture of polyvinyl alcohol and ferric chloride was deposited on a polyester support and allowed to evaporate. The PVA-FeCl.sub.3 was then suspended over a solution of pyrrole in ethanol. Under these conditions, polymerization of pyrrole occurred on the PVA-FeCl.sub.3 surface to produce a highly conducting, flexible laminate.
Also, pyrrole has been electrochemically polymerized onto an electrode covered with vinylidene fluoride-trifluoroethylene copolymer (P(VDF-TrFE)). Electrochemical polymerization of pyrrole was carried out in a one compartment cell containing an electrode covered with the copolymer. Polypyrrole was incorporated into the P(VDF-TrFE) film by beginning at the electrode surface and continuing through to the film surface. This process resulted in very flexible and stretchable conducting films.
A method has been devised to coat textiles with a uniform layer of electrically conducting polymer via an absorption process. Polyaniline and polypyrrole are solution-polymerized onto nylon and polyethylene terephthalate fabrics. Examination of the fabrics indicates that each individual fiber is encased with a smooth, coherent layer of the ICP.
Similarly, a method has been developed for making an electrically conductive textile material which is a textile material made predominantly of fibers selected from polyester, polyaniline, acrylic, polybenzimidazole, glass and ceramic fibers, wherein the textile material is covered to a uniform thickness of from about 0.05 to about 2 microns through chemical oxidation in an aqueous solution with a coherent, ordered film of an electrically conductive, organic polymer selected from a pyrrole polymer and an aniline polymer. Examination of such materials indicates that each individual fiber is encased or enveloped with a smooth, coherent layer of the ICP.
Ultra-thin films of emeraldine hydrochloride have been formed on poly(methyl methacrylate) (PMMA) and polystyrene (PS) substrates. The laminate films are formed by the oxidative polymerization of aniline at the interface between a lower oxidizing aqueous solution and an immiscible solution of the polymer and aniline monomer in chloroform. Volatilization of the chloroform yields a free-standing laminate film of the desired polymer substrate coated on one side with a continuous layer of emeraldine hydrochloride. These laminate films possess the mechanical properties of the substrate and exhibit conductivities in the region of 10 S/cm.
ICPs have been polymerized in the pores of microporous support membranes, yielding thin, conductive films on the membrane surface. In one process, a microporous membrane is used to separate solution of a heterocyclic monomer from a solution of a chemical oxidizing agent. As the monomer and oxidizing agent diffuse toward each other through the pores in the membrane, they react to yield conducting polymers. The result is an ultrathin film, electrically conducting composite polymer membrane.
An interfacial polymerization method has been developed in which the pores of a microporous support membrane are filled with an oxidative polymerization reagent. The membrane-confined solution is exposed to a vapor phase containing a monomer which can be oxidatively polymerized to yield a conductive polymer. A thin, defect-free film of the conductive polymer grows across the surface of the microporous support membrane.
Recently, strong and highly conductive films up to 0.6 mm thick have been formed from polyaniline gels. These gels are prepared from emeraldine base solutions in N-methyl-2-pyrrolidinone. The films are doped with a variety of doping agents. In terms of conductivity, mechanical properties, and thermal stability, methane sulfonic acid and ethane sulfonic acid dopants yield the best films.
Concerns about limited conductivity and constraints associated with efforts to increase conductivity through increased thickness have been addressed by earlier investigators. However, attempts to increase conductivity through mere increase in thickness of the conductive layer has been associated with poor abrasion resistance of the conductive layer, a tendency to undergo shear-induced delamination, and non-uniformity.
Surface phosphonylation has been achieved through a modified Arbuzov reaction using two approaches by Shalaby et al. in U.S. Pat. Nos. 5,491,198 and 5,558,517. In one approach gas phase phosphonylation is used to create acid-forming functional groups on surfaces in two steps. The first step entails chlorophosphonylation of a hydrocarbon moiety via the reaction of phosphorus trichloride (PCl.sub.3) and oxygen, which yields the corresponding phosphonic dichlorides. The phosphonyl dichlorides are subsequently hydrolyzed to phosphonic acid.
In the second approach, a liquid phase method for the surface phosphonylation of preformed thermoplastic polymers has been developed. The polymer is placed in a solution of 10% (v/v) PCl.sub.3 in carbon tetrachloride which is bubbled with oxygen. Additionally, a gas phase process for surface phosphonylation has been developed. In this method, the polymer is suspended in a flask containing several drops of PCl.sub.3 and oxygen gas. In each method, the polymer is quenched in water after allowing the reaction ample time to reach completion. Characterization of the polymers treated by each method indicates the presence of reactive phosphonate groups on their surface and no change in the bulk material properties.
Organic ICPs have been formed in-situ onto polymeric surfaces that have been chemically activated, such as by the phosphonylation processes described above, such that the ICPs are molecularly bonded to the substrate, as is described in U.S. Pat. No. 5,691,062 and U.S. Pat. No. 5,849,415, both of which are incorporated herein by reference. However, no precise means was taught for modulating the degree of electrical conductivity of the ultimate product.